The invention relates to a mask for use in forming elements on a structure and to a method of making a mask.
A mask of the above mentioned type can be used for ion etching, for ion implantation, and in X-ray, ion and electron beam lithography. For convenience, reference will mainly be made hereinbelow to the use of this mask in electron beam lighography only, however, the factors to be observed and the difficulties to be overcome are similarly typical for the other possible uses of this mask. This discussion with reference to electron beam lithography does not mean that the mask can not be used for other purposes.
For making structures with elements whose dimensions are in the .mu.m range and lower by means of "photo"-lithographic methods, ultraviolet light is no longer suitable for irradiation since it does not provide the necessary resolution. In such cases, the radiation-sensitive resist is preferably irradiated with electrons. Usually so-called electron beam pattern generators are used for this purpose. In these devices, the workpiece is coated with a radiation sensitive resist. The workpiece is then directly irradiated selectively by an electron beam whose diameter is smaller than the dimension of the smallest structural element in the structure to be made. The electron beam scans the resist layer in such a manner that at the end of irradiation each point of the resist layer has been in the beam path; the electron beam being blanked out during scanning in all those positions where the resist is not to be irradiated. With these devices, very complex patterns showing very small pattern elements can be transferred with satisfactory precision. However, there is the disadvantage that they are of a very complex structure and the output obtained is so low that electron beam scanning cannot be used practically under manufacturing conditions.
The output is higher when the resist layer on the workpiece is irradiated through a mask which is spaced from the resist layer by a small distance of less than about 500 .mu.m. The mask contains the pattern to be transferred in a ratio of 1:1. For transferring the pattern, the mask is first adjusted with respect to the workpiece, and then an electron beam whose diameter is not critical scans the mask line by line until each point of the mask has been irradiated. The mask can contain the entire pattern to be transferred, in which case the irradiated workpiece can be directed immediately to the next process step.
If the pattern to be transferred, consists of a large number of identical parts of a pattern it is possible to use a mask containing only one such part of the pattern. In this case, irradiation is effected in such a manner that upon the first alignment of the mask relative to the workpiece the part of the pattern is transferred. Subsequently, the mask is shifted relative to the workpiece by a distance corresponding at least to the width or the length of the part pattern, then the part pattern is again transferred, and these process steps are repeated until the desired number of parts of the pattern has been transferred to the radiation sensitive layer on the workpiece.
The prerequisite for transferring patterns with the necessary precision and the required high output is a mask which satisfies predetermined conditions. Where the mask is to be transparent to electrons it has to have throughholes for with every material, however thin, the electrons are scattered to such an extent that a pattern transfer with an exact edge definition in the .mu.m or sub-.mu.m range would be impossible if they have to pass through the material. The hole pattern in the mask has to represent the pattern to be transferred at least with the precision with which the pattern is to be transferred. Furthermore, the areas of the mask responsible for the transfer of the pattern, particularly the areas in the vicinity of the hole edges, have to be of the correct thickness. On the one hand, these areas have to be so thick that in those places where no electrons are to pass all electrons are readily absorbed. On the other hand, these areas are not to be too thick to avoid scattering effects at the pattern edges. These scattering effects prevent an exact definition of the pattern upon the transfer to the radiation sensitive layer. Finally, the mask has to be thermally stable since a high output can be reached only with a correspondingly high electron current which heats the mask.
Masks with throughholes to be used in electron beam lithography are known. L. N. Heynick, to give an example, has described a mask in the article "Projection Electron Lithography Using Aperture Lenses" published in the IEEE Transactions on Electron Devices, Vol. ED-22, No. 7. July 1975, pages 399ff. The manufacturing basis of this article is a silicon wafer carrying a thin chromium layer and a gold layer. The mask pattern is etched into the two metal layers and approximately 25 to 30 .mu.m deep into the silicon substrate. Subsequently, the silicon wafer is thinned from the back to such an extent that the mask holes are open also in the direction of the back. Although the finished mask satisfies the condition of showing throughholes it is impossible to reproducibly make masks having the correct thickness.
In the article "Fabricating Shaped Grid and Aperture Holes" by R. A. Leone et al. published in the IBM Technical Disclosure Bulletin, Vol. 14, No. 2, July 1971, page 417, the making of a silicon wafer by a mask with throughholes is described. There, a thin N-doped epitaxial layer is first grown on an N.sup.+ -doped silicon substrate. The mask pattern is generated in the epitaxial layer. Subsequently, the substrate is thinned from the wafer back, an etchant being used which selectively etches N.sup.+ -doped silicon in the presence of N-doped silicon. In this manner, the thickness of the remaining silicon layer--i.e. mainly the epitaxial layer--can be controlled easily and precisely. In order to improve the heat conductivity of the mask and its electron-absorbing properties it is furthermore suggested to cover the epitaxial layer with a thin gold layer.
Another mask with throughholes and its production is described in European patent application No. 0001038. Here, too, the basis of the mask is a silicon wafer. For the thinning of the silicon wafer from the back in a defined way after the mask pattern has been etched therein, a highly boron-doped surface layer of predetermined thickness is formed on the front of a wafer substrate having P-doping. The surface layer is resistant to the etchant subsequently used for thinning.
The two last named masks have the disadvantage that, if they are to transfer pattern elements with dimensions in the lower micron range, the mask may cause scattering effects distorting the pattern.
If the thinned area having the mask pattern has a relatively wide span, its thickness must not go below a predetermined value for reasons of the necessary thermal and mechanical stability. Thus, the thickness of the thinned area must not be reduced below the value of a few .mu.m if these mask areas are to span without any support over an area of several millimeters. However, the amount of scattering effects increases with the thickness of the edges of the holes defining the pattern.
It is the object of the invention to provide a mask for surface area structurization which permits the transfer of structural elements having their smallest dimensions in the .mu.m range and lower with a very high precision, with a high output being possible, as well as to provide a relatively simple process for making such a mask.
These and other objects will become more apparent from the following detailed description and the accompanying claims.
The mask according to the invention is mechanically stable even under thermal stress and also in those cases where the recesses to be formed are relatively large, and where consequently the mask area has a relatively wide span. The reason for such mechanic stability is that the structure includes a differently doped layer which can be relatively thick, without impaired precision of pattern transfer due to scattering effects for the mask as disclosed by the invention. The structure can also be used in those cases where the pattern to be transferred has elements with dimensions in the submicron range. It is in this aspect that the mask as disclosed by the invention differs from known masks where an increase of mechanic stability always involves an impaired transfer quality.
The above mentioned great advantage of the mask as disclosed by the invention is due to the fact that the mask pattern is designed in a metal layer or layers and extends laterally in a defined manner over the pattern in the doped layer. In the area of the edges of the holes in the metal layer the mask is so thin that no scattering effects are encountered. On the other hand, the mask can be designed by the method disclosed by the invention in such a manner that in the area of the hole edges in the metal layer the mask can be made of such a uniform thickness that even if the mask thickness approaches the minimum value necessary for a complete absorption of the electrons, the electrons do not penetrate the mask material in any point. If gold or a similar high density material is used as a metal, the metal layer can be less than 1 .mu.m thick when using a typical electron energy of 25 keV, without the risk of an incomplete absorption of the electrons. With the mask disclosed by the invention, pattern elements of any form can be transferred as long as they are not annular or of a closed loop shape. For transferring closed loop shaped pattern elements two masks are required. Further reference will be made thereto below.
Owing to its thermal stability, a high electron current (in the order of 50 to 100 .mu.A) does not affect the pattern transfer quality by distortion of the mask. The thermal stability of the mask and thus the output obtainable can be advantageously increased because the doped layer is highly boron-doped. The boron atoms introduced into the silicon lattice effect a tension in the foil formed on the doped layer during thinning. The tension is released only by a temperature increase to 120.degree. C. This tension effects on the one hand a high thermal stability of the mask as disclosed by the invention; on the other hand the high tension in the doped layer does not cause any critical distortions of the mask pattern (the whole pattern must not be distorted to more than 0.1 .mu.m). A monocrystalline substrate favors the forming of the advantageous non-distorting tension in the foil. With the boron-doped mask according to the invention, having a span of the thin mask section or foils of 6.times.6 mm, such a high electron current can be applied that, in 80 msec, a 5.times.5 mm surface can be irradiated with sufficient intensity. With the above mentioned electron beam pattern generator an irradiation period of more than 1 sec is required for the same surface.
A high output is also possible with the mask as disclosed by the invention in those cases where the mask is of such a size that it covers only part of a substrate to be irradiated, i.e. those irradiation processes where only part of the substrate can be irradiated, and where the process steps of shifting the mask relative to the substrate, aligning the mask relative to the substrate and repeating the irradiation several times.
The process suitable for making the mask as disclosed by the invention is easily controlled. The step of underetching the metal layer is particularly controllable in that it is easily possible to produce underetchings of fractions of a .mu.m reproducible within very small tolerances.
Another advantage of the method as disclosed by the invention is that apparatus, chemicals and some techniques used in semiconductor technology can be employed.